1. Field of the Invention
This invention relates in general to the field of x-ray optics and x-ray imaging and, in particular, to an x-ray interferometer useful for analyzing high density plasmas and optically opaque materials.
2. Description of the Problem
Diagnosis of plasma density and density variations is difficult when densities are high because high-density plasmas absorb most electromagnetic radiation. Density diagnosis by visible-light interferometry is therefore impossible for many types of experiments including inertial-confinement fusion (ICF) and ultra-high-irradiance plasma studies.
Possible areas where X-ray interferometry could be a useful diagnostic include: side-on and face-on radiography to measure density variations well above critical density such as shocked, cold, near-solid-p plasmas and USP laser irradiated solids; measurements of plasma formation near hohlraum walls a sub-critical densities to complement existing data; and measurements of ICF implosion density.
A related problem area is the diagnosis of opaque ICF beryllium shell targets containing cryogenic hydrogen ice. This ice layer must be extremely smooth, and characterization is made extremely difficult or impossible when the shell surrounding the ice is not transparent to visible light. One possible solution is x-ray interferometry, but the difficulty is utilizing relatively incoherent x-ray sources and controlling vibrations.
High-energy x-rays can penetrate these plasmas, but optical systems are difficult to fabricate, and path lengths must be extremely well matched in order to produce interference effects which relate to density. X-ray interferometers using reference beam paths have been produced, but are not suitable for laser-produced plasma experiments.
X-ray interferometry is difficult for several reasons. Optical path differences (OPDs) are small at low densities and at short wavelengths, which would imply that long wavelengths would perform better. For 2 keV photons for example, there is only an approximately eight wave phase difference for a 100 xcexcm path length through a vacuum compared to a similar path through a near-solid-xcfx81plasma target. However, adsorption is large at high densities and long wave lengths. In addition, longitudinal coherence lengths are small for x-rays, making path length matching challenging. Requirements for spatial coherence, backlight brightness, useable time resolution and target-plasma self-emission limits can also be stringent.
It is therefore an object of the present invention to allow application of optical techniques to x-ray interferometry.
Another object of the present invention is to provide x-ray interferometry imaging having matched path lengths for all rays to the 4th order in the light-path function.
Another object of the present invention is to provide x-ray interferometry having high-resolution imaging that allows fine-scale features to be probed.
A further object of the present invention is to provide X-ray interferometry having high monochromaticity.
Another object of the present invention is to provide X-ray interferometry wherein rocking-curve width is large near normal incidence.
These and other object are provided In the shearing interferometer of the present invention by using wavefronts created by a high-quality spherically-bent imaging crystal operating near normal incidence. A target is back-illuminated by a point-like x-ray source and the spherically-bent imaging crystal provides equal optical path lengths to one or more diffraction gratings acting as x-ray beam splitters. The split beams are analyzed to determine optical path length or phase differences between beams following different trajectories through the target. The spherically-bent imaging crystal enables broad-bandwidth x-ray sources to be utilized. The diffraction grating provides efficient x-ray beam splitting. And the common-path arrangement minimizes susceptibility to vibrations for static measurements.
